1. Field of the Invention
The present invention relates to an image-capturing element, an image-capturing circuit for processing an image-capture signal provided by an image-capturing element, an image-capturing device employing the image-capturing element and/or the image-capturing circuit, and a driving method of the image-capturing element.
2. Description of the Related Art
When obtaining a black and white image using an image-capturing element that captures a color image via primary color filters, i.e. red (R), green (G) and blue (B) color filters, or via complementary color filters, i.e., cyan (C), magenta (M) and yellow (Y) color filters in the prior art, a specific arithmetic operation for calorimetric system conversion is used on the primary color or complementary color element values of the individual pixels constituting the color image that has been obtained. In other words, in a manner similar to that adopted when achieving a color image, the image-capturing element outputs an image-capture signal and a specific conversion formula is used with a signal processing circuit or a signal processing software program on the output results to then convert the image-capture signal to a black and white image for output.
For instance, when converting a color image constituted of R, G and B elements to a black and white image, the color image is converted to a brightness Y in the YIQ colorimetric system adopted in the NTSC method. This conversion is expressed as;Y=0.299R+0.58G+0.114B, and is achieved by a signal processing circuit or a signal processing software program.
However, when a signal processing circuit converts the color image described above to a black and white image, a problem arises in that black and white image output cannot be obtained promptly since the color image-capture signal must first undergo A/D conversion to become converted to a digital signal, which must then be temporarily stored in memory and a significant length of time is required for the arithmetic operation in signal processing performed by the signal processing circuit or in the signal processing software to convert the color image to a black and white image.
In particular, in the case of an electronic camera that performs such processing, due to the fact that the black and white images cannot be output at high speed, high speed continuous photographing (continuous shooting) cannot be achieved.
In addition, in some color electronic cameras employing an IT-CCD (interline transfer CCD), electrical charges stored at pixels provided vertically adjacent to each other are added to double the electrical charge signal quantity. This achieves an improvement in sensitivity.
However, it is necessary to further increase the number of pixels to improve the sensitivity by adding the electrical charges at the pixels while maintaining the resolution at a specific level or higher using primary color filters i.e., red (R), green (G) and blue (B) filters and, at the same time, an increase in the number of pixels causes a problem in that the quantity of electrical charge that can be handled by each pixel is reduced due to restrictions related to the layout of the pixels and the electrical charge transfer path resulting in a reduced degree of sensitivity and a reduction in the dynamic range of the photoelectric conversion.
In addition, there are image-capturing devices having color filters with varying spectral transmittance characteristics provided at pixels at specific positions, which add together the pixel electrical charges and read out the results of the addition to obtain a color image using color filters with complementary color components.
Furthermore, there are image-capturing devices having color filters achieving the same spectral transmittance characteristics provided at pixels at specific positions, which add together the pixel electrical charges and read out the results of the addition to obtain a color image using color filters with primary color components.
However, a problem exists in that the electrical charge quantity that may be handled by a pixel on an image-capturing element. i.e., by a photoelectric conversion element, is within the range between the dark current level and the saturation level, and that, with this range greatly affecting the dynamic range of the photoelectric conversion characteristics, image-capturing cannot be achieved over a wide dynamic range.
In this case, even if the spectral transmittance factors at the color filters are increased to ultimately increase the sensitivity by increasing the electrical charge quantities, the dynamic range remains unchanged, and also, even if the spectral transmittance factors at the color filters are reduced to lower the sensitivity by reducing the electrical charge quantities, the dynamic range itself, likewise, remains unchanged.
In addition, a certain type of correlated double sampling (CDS) circuit or the like is connected to a two-dimensional solid image-capturing element such as a CCD, to reduce the reset noise, the output amplifier noise and the like of the two-dimensional solid image-capturing element included in the output signal from the two-dimensional solid image-capturing element.
In FIG. 25, which illustrates the structure of a CDS circuit in the prior art, the electrical charge that has undergone photoelectric conversion and has been transferred at an image-capturing element 401 is output as an image-capturing signal to a clamp circuit 406 via a buffer amplifier 402. The clamp circuit 406 outputs the output from the buffer amplifier 402 via a coupling capacitor 403, and also applies a voltage provided by a DC voltage source 405 to the output side of the coupling capacitor 403 by closing a switch 404 with the timing of a timing pulse øp. Thus, the image-capture signal from the coupling capacitor 403 is clamped at the field-through level to remove the reset noise, the output amplifier noise and the like mentioned above.
The image-capture signal output from the clamp circuit 406 is then output to a sample hold circuit 410 via a buffer amplifier 407. The sample hold circuit 410 samples the image-capture signal provided by the buffer amplifier 407 using a clamp pulse øs input to a switch 408, and outputs the level of the signal held at a holding capacitor 409 as an image-capture signal. Next, the image-capture signal indicating the signal level output by the sample hold circuit 410 is output via a buffer amplifier 411, is subsequently converted to a digital signal at an A/D converter (not shown) and desired image data that have undergone various types of signal processing are output.
Next, in reference to the timing chart in FIGS. 26A through 26C, the operation of the CDS circuit in FIG. 25 is explained. The image-capture signals output by the image-capturing element 401 via the buffer amplifier 402 are sequentially transferred and output as electrical charge signals that are achieved through conversion implemented by the individual photoelectric conversion elements at the image-capturing element 401 as indicated by the waveform of the image-capturing element output in FIG. 26A. In the image-capturing element output, the electrical charge quantity increases in units of pixels after the image-capturing element 401 is reset (R) and becomes stabilized at a signal level LL. After the reset (R), the clamp pulse øp is applied between a time point tt1 and a time point tt2, and the image-capturing element output is clamped at a field-through level LS that has been applied to match a reference level in which the noise is removed. After this, the signal level LL after clamping is sampled using the timing pulse øs during the time period elapsing from a time point tt3 to a time point tt4, and the signal level LL that has been held is output as an image-capture signal. In other words, the signal level LL relative to the field-through level LS is output.
Now, the output of the image-capturing element may be output with the electrical charge signal added at a floating diffusion amplifier (FDA) provided at the last stage of the image-capturing element to achieve an improvement in the sensitivity of the electrical charge signal and the like.
In such a case, two different signal outputs are output after a single reset, as illustrated in FIG. 20, which is to be detailed later, and a CDS circuit in the prior art presents a problem in that since it is capable of sampling only one electrical charge achieved through addition (or a single electrical charge that does not result from addition), the two different output electrical charges cannot be utilized effectively.
In addition, in order to obtain high resolution black and white (monochrome or monotone) images by employing an image-capturing element capable of obtaining color images through an image-capturing operation performed via primary color filters having color components, green (G), red (R) and blue (B), various types of interpolation processing are performed on pixel signals representing the individual color components that have been obtained to achieve a black and white image output.
For instance, a high resolution black and white image output is achieved through interpolation processing in which red (R) pixel signals and blue (B) pixel signals having undergone white balance processing are added and averaged or only green (G) pixel signals are used to be add and averaged for adjacent pixels in the horizontal and vertical directions.
However, the so-called “jaggies” may occur, in which fine lines and boundaries of colors in the image become jagged due to characteristics of the interpolation processing in a high resolution black and white image output obtained through the interpolation processing described above. In addition, jaggies may occur also due to a shift in the white balance. In either case, a poor black and white image with jaggies is output.
Furthermore, in order to obtain a high resolution black and white image through the interpolation processing described above, a dedicated signal processing circuit must be provided or it is necessary to install a special software program to perform the interpolation. The result is an increase in the scale of the circuit or an increase in the number of installed software programs. In either case, a great deal of time and effort is required and further miniaturization and further reduction in the weight of the image-capturing device are hindered.